ATLANTA — Thomas Harris slid into the cool, salty water of a 6.3-million-gallon tank at the Georgia Aquarium here and let himself float limp as kelp.

Mr. Harris, a former Army medic, gazed through a diving mask at a manta ray the size of a hang glider doing slow somersaults above shifting schools of silver fish. A 21-foot whale shark brushed silently by, inches from his face, its broad, spotted back taking up his entire view. Immersed in the moment, he forgot about the world.

This is not a weekend hobby. It is part of his therapy for the post-traumatic stress disorder he has been grappling with after his tours in Iraq. And like Mr. Harris, more veterans are turning to these sorts of outside-the-office treatment.

The broad acceptance of PTSD after the wars in Iraq and Afghanistan has posed an unexpected challenge. Acknowledging PTSD has only spurred a wide-ranging debate over the best way to treat it.

Traditional medical approaches generally rely on drugs and controlled re-experiencing of trauma, called exposure therapy. But this combination has proved so unpopular that many veterans quit before finishing or avoid it altogether. This has given rise to hundreds of small nonprofits across the country that offer alternatives: therapeutic fishing, rafting and backpacking trips, horse riding, combat yoga, dogs, art collectives, dolphin swims, sweat lodge vision quests and parrot husbandry centers, among many, many others.

A decade ago, mainstream psychiatry often dismissed these therapies. But now, as new studies suggest that things like yoga and interacting with animals can be as beneficial as drugs in reducing depression and anxiety without side effects or stigma, a growing number of psychotherapists are building them into treatment plans.

It is not hard to find veterans who had bad experiences with traditional treatment.

Mike Hilliard, the dive master who leads swims in the Georgia Aquarium’s huge tank, fought depression and anger after two combat tours, including a stint in Iraq in which he was shot in the head.

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Mike Hilliard, left, a dive master and former Army sergeant who did combat tours, instructing veterans before they swam in the whale shark tank at the Georgia Aquarium.CreditMelissa Golden for The New York Times

“Treatment had always been someone telling me I was dysfunctional and giving me a bunch of pills. I became more withdrawn to the point where I was considering ending it all,” said Mr. Hilliard, a former Army sergeant. He repeatedly dropped out of care, and found nothing that really helped until he tried scuba diving. “As soon as I was underwater, everything went quiet. Seeing the fish, hearing the ocean — there is a complete innocence about it. There are no bad memories in the water. Everything just wants to live. It made me want to live again.”

He now guides groups sent by the military and the Department of Veterans Affairs on a similar journey in the aquarium several times a week.

“It’s not just a ‘thank you for your service’ thing,” he told a group on a recent morning, moments before plunging in with the sea life. “This is a real tool you can use to rebuild your life.”

The crush of veterans seeking treatment beyond drugs and exposure therapy has pushed psychologists to try to scientifically evaluate programs that were once largely dismissed as field trips. But many psychiatrists are troubled by a lack of hard evidence supporting alternative therapies.

“Interest has just exploded,” Lt. Col. Gary Wynn, a psychiatrist who teaches at the Uniformed Services University of the Health Sciences, the military’s medical school. “I work with the V.A. and the military. There is no one who thinks this is just silly alternative medicine stuff anymore.”

But, he cautioned, evidence of real benefit is in many cases still often slim or nonexistent, in part because of a lack of funding for studies, and in part because alternative therapies are harder to assess than drugs.

“If I’m studying the benefits of fly fishing, do I control for the number of fish people caught? Or for the weather?” he said. “There is still a lot of work to be done on this.”

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Thomas Harris, an Iraq war veteran who has post-traumatic stress disorder, was fitted with breathing equipment before entering the shark tank.CreditMelissa Golden for The New York Times

PTSD was not formally recognized until 1980. In the relatively young field, people are still figuring out the best ways to treat it, said Dr. Bessel van der Kolk, a Boston-based psychiatrist and a best-selling author, who helped get the disorder recognized. The consensus seems to be shifting away from reflexively medicating patients, and toward complementing psychotherapy with things like yoga, he said.

“In the beginning I was very gung-ho on drugs. I did a lot of the early studies for drugs for PTSD. But we very quickly realized they don’t work very well,” he said. “Every veteran since Homer has been doping himself up to keep his issues under wraps, but it doesn’t help process the trauma.”

In the 2000s Dr. van der Kolk published one of the first studies about the effects of yoga on PTSD. “It had very good results. After eight weeks, six months, the positive effects are still there,” he said. He has since made yoga a core part of his practice.

Long before medical researchers began trying to document benefits, veterans sought out the healing potential of the world beyond the doctor’s office. The first personto walk the entire Appalachian Trail, Earl Shaffer, had just come home from World War II and told friends he needed to “walk the Army out of my system, both mentally and physically.” After Vietnam, hundreds of veterans sought refuge in the wilderness.

“Most of my work is being driven by the veterans,” said Daniel Libby, a Yale-trained psychologist who teaches yoga at a veterans center in Oakland, Calif., run by the Department of Veterans Affairs. “They don’t want to be on medications. Yoga offers a therapy that is self-empowering. You don’t have to rely on the medical system.”

In 2010, Mr. Libby surveyed the department’s health system and found that 28 percent of hospitals offered yoga. Now he estimates that it is more than 60 percent.

Dr. Barbara Rothbaum, a psychologist at Emory University who runs an intensive two-week PTSD treatment program, complements her traditional therapies with alternative ones.

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John Robert Walker, an Iraq war veteran, held onto the aquarium dock and acclimated himself to his gear as a whale shark swam by.CreditMelissa Golden for The New York Times

She recently helped compile a National Academy of Sciences report on therapies for PTSD. “We met a lot of well-meaning clinicians around the country creating programs with equine therapy or wilderness therapy or whatever, and there was no way to know if any of it worked,” she said. “Because of that, we couldn’t recommend it.”

Dr. Rothbaum relies on exposure therapy in her clinic. At the same time, though, she has added yoga and meditation to help patients relax.

“It’s not a core treatment, but it’s a life skill that helps maintain the gains,” she said.

She also sends patients to places like the Georgia Aquarium to challenge them to overcome fears and build new experiences that put traumatic memories in perspective.

After 30 minutes, Mr. Harris, the former Army medic, emerged from the water with a broad grin. “Amazing,” he said.

When Mr. Harris got out of the Army, he went to the Department of Veterans Affairs several times for therapy and medication, but always dropped out in frustration and threw his pills away.

“I didn’t want to have to take meds. I’ve always been a cornerstone, helping other people,” he said. “I guess I was mad at myself.”

But nightmares and violent outbursts grew worse. One day he blew up at his sister over a vacuum cleaner cord left out in a hallway and slapped her. Shocked at his actions, he vowed to give therapy another try.

He now does a combination of traditional exposure therapy and medication, but also meditation and yoga, which, he said, “makes you feel better all day.”

Swimming with the sharks, to him, was more than a field trip.

“It’s relaxing, sure. It made me feel great. But it’s also kind of scary, with those big sharks,” he said. “And part of therapy is doing what you have been avoiding, overcoming your fears.”

Correction:

An article last Sunday about alternative therapies for post-traumatic stress syndrome misstated the academic credentials of Dr. Barbara Rothbaum, who runs a treatment program at Emory University in Atlanta. She is a psychologist, not a psychiatrist.